Altermagnetic signatures and topological textures in ultrathin multiferroic BiFeO₃

Nature Materials (2026) Cite this article 25 AltmetricMetrics detailsMagnetoelectric multiferroics promise low-power switching, non-volatile memory and signal transduction, yet practical deployment requires ultrathin films in which polar and magnetic order often degrade. Uncompensated bound charge drives depolarization in ferroelectrics, whereas off-stoichiometry, reduced anisotropy and charge transfer can produce magnetic dead layers. Here we realize a multiferroic phase of BiFeO3 with no dead layer, revealing signatures of emergent altermagnetism in the ultrathin limit. First-principles calculations, symmetry analysis, atomic-resolution imaging and magnetic imaging show that short-circuit electrostatic boundary conditions and epitaxial strain drive a continuous, thickness-induced phase transition accompanied by multiferroic topological textures. At four unit cells, these boundary conditions stabilize a collinear monoclinic phase with d-wave altermagnetic order, supported by magnetic circular dichroism and spin-resolved photoemission measurements. These results establish a pathway to stabilize unconventional multiferroicity at device-relevant thicknesses, reframing scaling limits for oxide electronics. 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This research used resources of the Advanced Light Source, a US Department of Energy, Office of Science User Facility, under contract number DE-AC02-05CH11231. This work made use of the synthesis facility of the Platform for the Accelerated Realization, Analysis and Discovery of Interface Materials (PARADIM), which is supported by the National Science Foundation under cooperative agreement number DMR-2039380. We thank T. Butcher and M. Furo for valuable discussion.Scanning NV magnetometry measurements were supported by the Air Force Office of Scientific Research under FA9550-24-1-0169 (L.C.). BiFeO3 synthesis and measurements were supported by NSF EPSCOR RII Track-4 Research Fellows Program under OIA-2327352 (L.C.) and under ETHOS MURI via cooperative agreement number W911NF-21-2-0162 (M.R., D.G.S., R.R). X.L. acknowledges support from the Rice Advanced Materials Institute (RAMI) at Rice University as a RAMI Postdoctoral Fellow. X.L., Y.H. and R.R. acknowledge support from NSF (FUSE-2329111 and CMMI-2239545) and Welch Foundation (C-2065). X.L. and Y.H. acknowledge the Electron Microscopy Center, Rice University. S.M.U.S., J.S. and A.W. acknowledge funding from the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) via projects 268565370 (SFB TRR173 projects A09 and B14) and 465145163 (CRC 1552 project A02). L.Š. acknowledges funding from ERC Starting grant number 101165122 and DFG grant number TRR 288-7422213477 (Projects A09 and B05). QPDI-based PFM measurements were conducted as part of a user project at the Center for Nanophase Materials Sciences (CNMS), which is a US Department of Energy, Office of Science User Facility, Oak Ridge National Laboratory. Y.N., S.P and L.B. also acknowledge support from the Vannevar Bush Faculty Fellowship (VBFF) via grant number N00014-20-1-2834 from the Department of Defense, ARA Impact Grant 3.0, the MonArk NSF Quantum Foundry supported by the National Science Foundation Q-AMASE-I Program under NSF award number DMR-1906383 and ARO grant number W911NF-25-1-0223. Spin-ARPES work at Cornell was supported by NSF DMR-2104427 and AFOSR FA9550-21-1-0168. We thank MAX IV Laboratory for time on beamline MaxPEEM under proposals 20230367, 20240599, 20241342, 20250678, 20250994 and 20251711. Research conducted at MAX IV, a Swedish national user facility, is supported by the Swedish Research Council under contract number 2018-07152, the Swedish Governmental Agency for Innovation Systems under contract number 2018-04969 and Formas under contract number 2019-02496. M. Congdon acknowledges that this material is based upon work supported by the National Science Foundation Graduate Research Fellowship Program under grant number 2439559. Any opinions, findings and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the National Science Foundation. This work was also supported by JSPS KAKENHI grant numbers 25K00961, 25K07211 (A.H.), the project Quantum materials for applications in sustainable technologies (QM4ST), funded as project number CZ.02.01.01/00/22 008/0004572 by Programme Johannes Amos Commenius, call Excellent Research (J.K.) and by the Ministry of Education, Youth and Sports of the Czech Republic through the e-INFRA CZ (ID 90254) (A.H. and J.K.). Part of the computations in this work were performed using the facilities of the Supercomputer Center, the Institute for Solid State Physics, the University of Tokyo.These authors contributed equally: Geo Fratian, Maya Ramesh, Xinyan Li.School of Engineering, Brown University, Providence, RI, USAGeo Fratian, Arundhati Ghosal, Morgan Congdon, Alexander Qualls & Lucas CarettaDepartment of Materials Science and Engineering, Cornell University, Ithaca, NY, USAMaya Ramesh, Evan Krysko & Darrell G. SchlomDepartment of Materials Science and Nanoengineering, Rice University, Houston, TX, USAXinyan Li, Shashank Kumar Ojha & Yimo HanRice Advanced Materials Institute, Rice University, Houston, TX, USAXinyan Li, Shashank Kumar Ojha & Yimo HanMAX IV Laboratory, Lund, SwedenEvangelos GoliasSmart Ferroic Materials Center, Physics Department, Arkansas Materials Institute, University of Arkansas, Fayetteville, AR, USAYousra Nahas, Laurent Bellaiche & Sergei ProkhorenkoInstitute of Physics, Johannes Gutenberg-University Mainz, Mainz, GermanySebastian Maria Ulrich Schultheis, Julian Skolaut & Angela WittmannCenter for Nanophase Materials Sciences, Oak Ridge National Laboratory, Oak Ridge, TN, USAMarti ChecaPlatform for the Accelerated Realization, Analysis, and Discovery of Interface Materials (PARADIM), Cornell University, Ithaca, NY, USAEvan Krysko & Brendan D. FaethDepartment of Physics, Laboratory of Atomic and Solid State Physics, Cornell University, Ithaca, NY, USAPaul Malinowski, Brendan D. Faeth & Kyle ShenMax Planck Institute for the Physics of Complex Systems, Dresden, GermanyJan Priessnitz, F. C. Fobasso Mbognou & Libor ŠmejkalDepartment of Physics and Electronics, Graduate School of Engineering, Osaka Metropolitan University, Osaka, JapanAtsushi Hariki, Kosuke Sakurai & Mizuki FuroDepartment of Physics, Brown University, Providence, RI, USAShiyu Zhou & Lucas CarettaExperimental Physics V, Center for Electronic Correlations and Magnetism, University of Augsburg, Augsburg, GermanyKai LitziusAdvanced Light Source, Lawrence Berkeley National Laboratory, Berkeley, CA, USAChristoph KleweDepartment of Materials Science and Engineering, University of California, Berkeley, CA, USAPeter Meisenheimer & Ramamoorthy RameshInstitute of Condensed Matter Physics, Faculty of Science, Masaryk University, Brno, Czech RepublicJan KunešKavli Institute at Cornell for Nanoscale Science, Cornell University, Ithaca, NY, USAKyle Shen & Darrell G. SchlomDepartment of Materials Science and Engineering, Tel Aviv University, Ramat Aviv, IsraelLaurent BellaicheMax Planck Institute for Chemical Physics of Solids, Dresden, GermanyLibor ŠmejkalInstitute of Physics, Czech Academy of Sciences, Praha, Czech RepublicLibor ŠmejkalLeibniz-Institut für Kristallzüchtung, Berlin, GermanyDarrell G. SchlomSmalley-Curl Institute, Rice University, Houston, TX, USAYimo HanDepartment of Physics, University of California, Berkeley, CA, USARamamoorthy RameshDepartment of Physics, Northeastern University, Boston, MA, USAPaul StevensonSearch author on:PubMed Google ScholarSearch author on:PubMed Google ScholarSearch author on:PubMed Google ScholarSearch author on:PubMed Google ScholarSearch author on:PubMed Google ScholarSearch author on:PubMed Google ScholarSearch author on:PubMed Google ScholarSearch author on:PubMed Google ScholarSearch author on:PubMed Google ScholarSearch author on:PubMed Google ScholarSearch author on:PubMed Google ScholarSearch author on:PubMed Google ScholarSearch author on:PubMed Google ScholarSearch author on:PubMed Google ScholarSearch author on:PubMed Google ScholarSearch author on:PubMed Google ScholarSearch author on:PubMed Google ScholarSearch author on:PubMed Google ScholarSearch author on:PubMed Google ScholarSearch author on:PubMed Google ScholarSearch author on:PubMed Google ScholarSearch author on:PubMed Google ScholarSearch author on:PubMed Google ScholarSearch author on:PubMed Google ScholarSearch author on:PubMed Google ScholarSearch author on:PubMed Google ScholarSearch author on:PubMed Google ScholarSearch author on:PubMed Google ScholarSearch author on:PubMed Google ScholarSearch author on:PubMed Google ScholarSearch author on:PubMed Google ScholarSearch author on:PubMed Google ScholarSearch author on:PubMed Google ScholarSearch author on:PubMed Google ScholarSearch author on:PubMed Google ScholarL.C. led study and guided the team. G.F., E.G., J.S., K.L., C.K., M. Congdon, A.W. and L.C. performed all the X-ray spectroscopy and imaging measurements. X-ray spectroscopy was analysed by G.F., E.G., S.M.U.S. and A.W., under the guidance of L.C. and A.W. M.R. optimized the synthesis of BFO thin films and performed the X-ray diffraction studies under the guidance of D.G.S. and L.C. X.L. performed all the STEM measurements and analysis under the supervision of Y.H. A.G. and M.R. performed the NV magnetometry and PFM with the help of S.K.O., S.Z. and A.Q., under the supervision of R.R., P.S. and L.C. QPDI-based PFM was performed by M. Checa. First-principles calculations were performed by S.P. and Y.N. under the guidance of L.B., as well as J.P. and F.C.F.M. under the supervision of L.Š. The XMCD calculations were performed by A.H., K. Sakurai, M.F. and J.K. M.R., P. Malinowski, E.K. and B.D.F. performed and analysed the spin-ARPES measurements under the supervision of K. Shen. The manuscript was prepared by L.C., A.W., G.F., E.G., P. Meisenheimer, P.S., M.R., X.L., L.Š., S.P., L.B. and R.R. All authors contributed to the discussion of the data in the manuscript and the Supplementary Information.Correspondence to Angela Wittmann or Lucas Caretta.The authors declare no competing interests.Nature Materials thanks Zhenxiang Cheng, Juraj Krempasky and Pu Yu for their contribution to the peer review of this work. Peer reviewer reports are available.Publisher’s note Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.a) Bragg scans for all samples ranging from 1.6 nm to 50 nm in BFO thickness. b) Reciprocal space maps around the {332}O family of substrate peaks (indicated by *) for the 50 nm sample with one BFO peak and two peaks corresponding to the two-ferroelectric domains in this orientation. Both SRO and BFO peaks have the same Qx value indicating they have the same in plane lattice spacing.a) Optical Images at different interferometer spot positions over the cantilever: 1 before, 2 on, and 3 after the “null” spot, which nullifies the in-contact resonance frequency peak during the sweep experiment in b,c) PFM amplitude as a function of AC bias. Blue points considered for the linear fit (blue line). Black points are discarded as they are below the instrument noise floor ( ~ 300 fm). d) PFM on-field loop (black is amplitude and red is phase) e) PFM off-field loop (black is amplitude and red is phase). f) Statistics of 256 PFM on-field PFM amplitude loops at different sample positions (blue line shows the mean loop). g) Statistics of 256 PFM off-field PFM phase loops at different sample positions (blue line shows the mean loop).a) DFT supercell showing the relaxed Mc structure of the BFO/SRO bilayer. Left and right panels show supercell views along [010]PC and [100]PC zone axes, respectively. Panels b) and c) show the comparison of computed polar displacements with experimental data presented in Fig. 1 of the Main Text. The Exp. 1 and Exp. 2 curves correspond to measured out-of-plane displacements shown in Main Text Figs. 1c and 1d, respectively. The purple curves in both panels are DFT results for the relaxed Cm phase.In-plane XLD histograms acquired at the Fe L3 edge for a) 1.6 nm, b) 3.6 nm, and c) 50 nm BFO, respectively. At the Fe L3 edge, the contrast contains contributions from both the ferroelectric (polar) order (color scale: red/blue) and the antiferromagnetic (AFM) order (purple/white). With increasing thickness, the dominant contribution evolves from AFM-dominated to polar-dominated, consistent with the polar vector rotating into the film plane (to which the beam is sensitive) and the AFM vector rotating out-of-plane (to which the beam is comparatively insensitive); see also Extended Data Fig. 6. Corresponding histograms at the Fe L2 edge for d) 1.6 nm, e) 3.6 nm, and f) 50 nm thick BFO. The Fe L2 edge predominantly reflects AFM order. In the thinnest films, the AFM vector lies in plane (beam-sensitive) and therefore dominates the signal, while the polar vector is largely out-of-plane (beam-insensitive). In thicker films, although the polar response is intrinsically suppressed at the L2 edge, the apparent polar-like signal increases as the AFM vector rotates out-of-plane (reducing its contribution), leaving the residual in-plane polar component more visible. Pseudo-vector maps at the O K edge (Methods) for g) 1.6 nm, h) 3.6 nm, and i) 50 nm BFO, which are selective to polar order only (red/blue) at all thicknesses. Red outlines on the histograms indicate large fit uncertainties or regions omitted from the vector map due to low signal, as in the O K-edge, 1.6 nm sample where the beam is sensitive to in-plane polarization, but the film’s polarization is predominantly out-of-plane. Finally, note that the purple/white asymmetry in the Fe L2, 1.6 nm panel is an image-processing artifact arising from the exclusion of high-error pixels.In plane X-ray linear dichroism (XLD) vector maps acquired at the a) Fe L3, b) Fe L2, and c) O Kedges. Single-angle XLD images with the incident linear polarization oriented \(\phi \,=\,45\)° relative to the [\(1\bar{1}0\)]PC direction (white inset arrow indicates polarization) at the d) Fe L3, e) Fe L2, and f) O K edges. This geometry predominantly probes ferroelectric (polar) order in BFO. Note the strong ferroelectric domain contrast at the Fe L3 and O K edges, and the absence of ferroelectric contrast at the Fe L2 edge. Single-angle XLD images with polarization \(\phi \,=\,90^\circ\) relative to the [\(1\bar{1}0\)]PC direction at the g) Fe L3, h) Fe L2, and i) O K edges, a geometry that predominantly probes antiferromagnetic (AFM) order. Here, robust AFM contrast appears at the Fe L3 and Fe L2 edges - particularly at ferroelastic domain boundaries - while no contrast is observed at the O K edge. All Fe-edge XLD images were obtained via energy-dependent XLD methods, whereas O-K-edge XLD images were acquired using opposite linear polarizations (see Methods). j) Cartoon plot showing the edge sensitivity in BFO. Scale bars are 1 µm.a) \({\left[100\right]}_{{\rm{PC}}}\) grazing incidence Fe L2 edge XLD intensity as a function of temperature for 100 nm (red), 1.6 nm (blue), and 0.8 nm (purple) BFO films grown on SRO (20 nm)//DSO. b,c) XAS intensity for various X-ray angles of incidence and for s- and p- light polarization. While XLD intensity (difference in intensity between s- and p-polarized light) decreases with decreasing BFO thickness for grazing incidence along [010]PC shown in (b), XLD intensity is strong with decreasing thickness for grazing incidence along \({\left[100\right]}_{{\rm{PC}}}\) and normal incidence geometries shown in (c).a) Topological defect density as a function of BFO film thickness, expressed in defects per µm2. Representative multiferroic textures observed in b) a 3.5 nm-thick BFO film, c) a 5 nm-thick BFO film, and d) a 10 nm thick BFO film. The 5 nm and 10 nm films exhibit a visibly reduced defect density compared to the 3.5 nm sample. All scale bars represent 1 µm.a) Valence band spectra (left) and integrated photoemission intensity (right) centered at Γ (cut #1). The Fermi level is fixed with reference to the underlying SrRuO3 buffer layer, with a valence band maximum near EB = 1.3 eV. Cut #1 is taken with the analyzer slits and ky defined parallel to [001]O. b) Spin-integrated iso-energy map at EF = 1.35 eV, just below the valence band maximum. c) Spin-polarized iso-energy map covering the k-space area circled in (b). Color scale indicates the x-component of the measured in-plane polarization Px. The right-most panel shows Px switching through the momentum-space region indicated by Cut #2, centered at kx = -0.15 Å−1.Simulated Fe L2,3-edge XAS (black) and XMCD (red) spectra for three Néel-vector orientations, a–c) L = [100]PC, d–f) L = [010]PC, and g–i, L = [001]PC. The spectra are shown for X-ray wave vectors k = [100]PC (a,d,g), k = [010]PC (b,e,h), and k = [001]PC (c,f,i).X-ray absorption spectroscopy (XAS) and real-space imaging reveal the spectral dependence of the magnetic contrast. a) Total XAS spectrum across the Fe \({{\rm{L}}}_{3}\)- and \({{\rm{L}}}_{2}\)-edges. b) Calculated XMCD spectra for Cm BFO with relativistic and non-relativistic contributions (purple) included, with only non-relativistic contributions (blue) included, and with only relativistic contributions included (red). c-g) Corresponding real-space XMCD-PEEM images acquired at precise photon energies noted in (a) and (b). Outlined yellow and red regions highlight some regions of interest within [100]PC-oriented antiferromagnetic domains, where XMCD contrast is expected via symmetry arguments (Supplemental Discussion IV, Extended Data Fig. 9). Experimentally, panels (c) and (e) exhibit a clear contrast reversal, with panel (d) yielding vanishing contrast as only expected from the nonrelativistic XMCD theory in (b). This mechanism is further confirmed in (f) and (g), where distinctive contrast reversal is observed despite the relativistic (canted moment) XMCD component predicting a negligible or unipolar contribution at those energies (that is, the XMCD contribution from the canted moment would not flip sign). Note the absence of reversal in domains where the Néel axis is along the \({\left[010\right]}_{{\rm{pc}}}\) direction as predicted by theory. h) XLD vector map of the antiferromagnetic domain orientation in the same region. Scale bars are 0.25 µm.Supplementary Figs. 1–19, Tables 1 and 2 and Discussions I–VIII.Springer Nature or its licensor (e.g. a society or other partner) holds exclusive rights to this article under a publishing agreement with the author(s) or other rightsholder(s); author self-archiving of the accepted manuscript version of this article is solely governed by the terms of such publishing agreement and applicable law.Reprints and permissionsFratian, G., Ramesh, M., Li, X. et al. Altermagnetic signatures and topological textures in ultrathin multiferroic BiFeO3. Nat. Mater. (2026). https://doi.org/10.1038/s41563-026-02786-1Download citationReceived: 14 January 2026Accepted: 05 October 2026Published: 28 October 2026Version of record: 28 October 2026DOI: https://doi.org/10.1038/s41563-026-02786-1Anyone you share the following link with will be able to read this content:Sorry, a shareable link is not currently available for this article. 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